1. Field of the Invention
The present invention relates to a method of manufacturing piezoelectric elements, which have three-dimensional surfaces for distribution of big masses at centers.
2. Description of Related Art
Piezoelectric elements, represented by quartz oscillators, have been used in various fields such as oscillation sources of standard frequency and clocks of electronic or electrical devices. Recent researches and developments are focused on how to reduce thickness of piezoelectric material and how to shape piezoelectric material to lenticular profiles for advanced performance of data processing or transmission capability.
As for large-scaled oscillators with electrodes of several millimeters or more in diameter, piezoelectric material is etched to domed shapes by a wet process and then dressed to profiles with curved surfaces by mechanically polishing edges of the domed shapes. As for small-scaled oscillators with electrodes of 1 mm or less in diameter, piezoelectric material is concaved to profiles suitable for fabrication of high-quality oscillators with less support loss.
A dry process (JP 2002-368572 A), wherein piezoelectric material is machined to an intermediate profile similar to a product shape and then dry etched to the product shape, is also proposed as an advanced concaving method. Other known techniques are a micro-processing method using a micromold or reflow (Li L. et al, Tech. Digest of Transducers (2003), pp. 508-511), and a dry process (JP 2003-234632 A). According to the dry process, a photosensitive agent is applied to a surface of the piezoelectric material at first. The photosensitive agent is then shaped to a photomask with predetermined thickness distribution by controlling an energy of light, which transmits through the photomask. Thereafter, piezoelectric material is dry etched to a product shape together with the thickness-controlled photomask.
Oscillators produced by these known methods are useful as quartz crystal microbalances (QCM) for detecting adsorption of molecules as changes in resonance frequencies. Multichannel-type QCMs are also provided by alignment of such quartz oscillators in array.
A domed profile, which can be shaped by a thermal reflow method, is limited to 100 μm or so in diameter at greatest, since thermal dynamics allows mass transport in a distance of several ten micrometers at most. Piezoelectric material is of course shaped to a bigger domed profile by use of a resist film of several ten micrometers or more in thickness, but such a thicker resist film is more irregular in thickness. Irregularity of the thickness distribution degrades a product profile. The other process, wherein predetermined thickness distribution is imparted to a resist film by controlling an energy of light, is extremely expensive for deposition of a desired mask in comparison with a conventional process, so that its applicability is limited from an economical point of view.